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A209401
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Number of noncommutative rings with n elements.
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4
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0, 0, 0, 2, 0, 0, 0, 18, 2, 0, 0, 4, 0, 0, 0, 228, 0, 4, 0, 4, 0, 0, 0, 36, 2, 0, 23, 4, 0, 0, 0
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OFFSET
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1,4
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COMMENTS
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a(n)=0 if and only if n is squarefree.
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LINKS
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Table of n, a(n) for n=1..31.
Gregory Dresden, Rings of Size Four, demonstrating that a(4)=2.
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FORMULA
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a(n) = A027623(n) - A037289(n).
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EXAMPLE
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For n=8, there are 52 rings of order 8, 18 of which are noncommutative, so a(8)=18.
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CROSSREFS
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Cf. A027623, A037289, A037291.
Sequence in context: A283950 A342376 A277443 * A029696 A118887 A057383
Adjacent sequences: A209398 A209399 A209400 * A209402 A209403 A209404
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KEYWORD
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nonn,nice,hard,more
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AUTHOR
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Ben Branman, Mar 26 2012
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STATUS
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approved
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